The present invention relates to sanitary or household absorbent papers, and in particular to a sheet consisting of at least one embossed ply and foremost used as toilet paper, though also applicable to handkerchiefs, napkins and paper towels.
As regards the manufacturing industry for sanitary and household papers, the material used is a low-weight and generally creped paper called cellulose cotton or wadding paper or tissue paper. Because of its stretchability, illustratively made possible by the creping, the sheet lends itself to being embossed. The embossing operation permanently deforms certain sites to produce protrusions on one side which correspond to recesses on the other side.
Regarding sanitary products, the trend of recent years has been to make them softer and more resilient by controlling thickness and strength, in particular by embossing. Such embossing also improves product appearance. Embossing is carried out either on high-moisture paper, i.e., within the wet portion of the papermaking machine, or on low-moisture paper, i.e., in the conversion stage. In the latter case, a dry sheet wound off a master reel in the papermaking machine will be processed. The present invention relates to the paper converted in the dry portion.
The most commonplace embossing patterns constitute a geometric repetition of elementary protrusions of small cross-sections and assuming simple geometric shapes. One embodiment is disclosed in U.S. Pat. No. 3,414,459 which relates to a stratified sheet of several elementary foils, called plies, which are bonded to one another. The plies are embossed at a protrusion density and height to attain water-absorbing products, for example, paper towels. The number of elements ranges from 5 to 30 per cm2. Applicant moreover has developed, in particular regarding sanitary papers, certain patterns with a higher number of elements ranging from 30 to 80 per cm2. These elements subtend very small individual top surfaces less than 1 mm2. Such latter embodiments imitate the looks of woven products. One such example is described in European Patent 426,548. This kind of embossing mainly affects the thickness of such a sheet. However, its aesthetics is modest.
In order to improve visual attractiveness, patterns have already been proposed of which the protrusion rate is much lower than in the above cited patterns, preferably being less than 2,000 elements per m2, and wherein the individual surface of the pattern-constituting protrusions is higher. The elements can fit into a polygon of 2 to 6 cm2. European Patent 265,298 describes one embodiment mode. In this Patent, cross-sections of the element-forming protrusions are elongated and their peaks are linear. The curves so subtended preferably are closed and impart a raised-fabric look to the pattern elements. Moreover, as the elements are comparatively spaced far apart, they subtend wide and smooth zones of which the smoothness is unaffected by embossing. This set of pattern features imparts customer-perceived softness and resiliency to the paper. On the other hand, this kind of pattern only modestly affects the paper""s physical properties, in particular, the thickness changes little and it was found that embossing efficacy is less. In order to gain thickness, the cylinders must be engraved at a relatively substantial depth.
Applicant in its French Application 94 15196 of Dec. 16, 1994 proposed combining the advantages of the two embossing modes to implement a sheet presenting both a graphics pattern composed of linear protrusions and a background pattern comprising small protrusions. The width of the linear protrusions, which in general are mutually apart, is between 0.1 and 2 mm. In general, the small protrusions are frustoconical and they are configured at a density of at least 30 per cm2. The background pattern provides the main part of the functional features relating to embossing, namely thickness and absorption in particular. Moreover, the high density of the background pattern""s protrusions leads to an eye-pleasing, textile appearance.
The background pattern""s protrusions emphasize the main pattern because they are hardly visible to the naked eye on account of their small size. Their surface at the top is less than 1 mm2. Such a contrast is achieved in particular by the high ratio between the surface of an element of the graphics pattern to that of a small protrusion, which is higher than 50, and by the high number of small protrusions per unit surface.
Nevertheless, when manufacturing a sheet made according to this Patent, the graphics pattern, which is salient relative to the cylinder, is pressed against the embossing rubber just as are the tips of the background pattern. Accordingly, there is competition between the two types of embossing. In particular, because of their small size, the tips provide more embossing than the graphics pattern. As a result, the graphics pattern loses marking and resolution qualities.
Moreover, because the protrusions are frustoconical, a neutral transition zone exists between the background pattern and the graphics pattern. The contours in this zone are fuzzy since it is generally determined by the tips in the rim zone according to a geometric pattern and by the free-shape contours of the graphics pattern.
The object of the invention is to create a sheet of absorbent paper which, in particular as compared to the latter embodiment above, offers improved pattern resolution and emphasizes the background pattern.
The invention relates to a sheet of absorbent paper, in particular of tissue paper, and of a specific surface weight of 10 to 40 g/m2. The sheet comprises a first embossed zone of the kind provided by embossing being carried out between an undeforming cylinder fitted with protrusions and a cylinder fitted with a resilient cladding, such as rubber. Accordingly, the sheet comprises on one side salients of which the tops are substantially situated in a plane P. Recesses corresponding to the protrusions are present on the opposite side. The sheet is characterized in that the first embossed zone constitutes a background base pattern of which the number of salients of height H1 relative to the plane P is larger than 30 per cm2 and in that the sheet further comprises at least a second, so-called graphics pattern constituted of salient-free zones and of a height H2 relative to the plane P which is larger than H1.
Surprisingly, it was found that on the side including the recesses, i.e., the side intended for the user, the graphics pattern offers a marked contrast relative to the background base pattern provided the graphics pattern is offset in height from the background. This solution is indeed contrary to that disclosed in French Application 94 15196 wherein the graphics pattern is defined by recess zones such as constitute the background base pattern.
The absorbent sheet of paper of the invention can include a single thickness, or one ply. However, the invention also relates to designs where the sheet includes several jointly embossed tissue paper plies.
H1 and H2 are mean values calculated using a number of appropriate statistical measurements.
In particular, the patterns shall be well defined when, following embossing, the sheet is sufficiently deformed so that H2 is 1.2 times H1.
In another feature of the invention, the mean diameter of disks which fit into the salient-free zones of the graphics pattern shall exceed the mean diameter of disks which fit between the salients of the base zone, preferably the former diameter being at least twice, and in particular, three-fold the latter.
A disk is considered to fit into a given surface portion if it is inside the portion while tangent to three of its salients.
In another feature of the invention, the second pattern includes at least two adjacent salient-free zones which are bounded from each other by isolated salients.
A salient is considered xe2x80x9cisolatedxe2x80x9d if the mean height H of the sheet portion enclosing it is greater than the height H1 of the background zone. In this manner the salient differs from a base pattern salient of which the mean height of the sheet enclosing it is exactly H1.
In a preferred embodiment of the invention, the salients constitute an alignment.
In another feature of the invention, the dimensions of the tops of the isolated salients are the same as those of the salients of the first zone.
This design provides the advantage of greater pattern unity and it enhances contrast because smaller salients of the embossing cylinder concentrate stresses and more sharply mark the sheet at the time of embossing. Lastly, engraving is easier than if the protrusions were of different dimensions. A single tool set can be used.
In another feature of the invention, salient-free zones of the second pattern include at least one calendered surface portion. Calendering results from the rubber cylinder resting against the bottom of the engraving. Thereupon roughness suppression smoothes the surface. Calendering improves softness, especially if this portion is large enough to favor touch. For that reason the calendered surface portion is at least 1 cm2, corresponding substantially to the skin portion of the finger tip making contact with the gripped sheet.
In another feature of the invention, the sheet of absorbent paper comprises two plies of which at least one shall be one of the absorbent plies. The absorbent plies are configured onto each other with the salients inside the structure. The two plies can be assembled in an arbitrary manner, for example, by knurling or bonding. The latter operation includes xe2x80x9chot meltxe2x80x9d or cold gluing using a PVA-type adhesive. The plies can be mutually offset in order that the graphics patterns shall coincide less than perfectly, and in this manner, the plies shall be partly combined. In this manner, the combination can be modulated and the sheet can be stiffened or made more flexible depending on the number of tip-to-tip combination zones that are left. Lastly, the plies can be fitted with different patterns.
The present invention also relates to the apparatus and method allowing manufacture of the sheet of the invention.
The apparatus is characterized in that it comprises at least one undeforming cylinder comprising a first zone of at least 30, in particular frustoconical, protrusions per cm2 and at least one graphics pattern zone devoid of protrusions and of which the depth relative to the protrusion tops is at least equal to that of the first zone, the mean diameter of disks fitting into the second zone being greater than that of disks fitting into the first zone.
In another feature of the invention, two second graphics pattern zones are adjacent but mutually separated by isolated protrusions.
In yet another feature of the invention, the apparatus comprises a cylinder fitted with a resilient cladding and cooperating with an undeforming cylinder, its resiliency being such that, in operation, the resilient material can make contact with the bottom of the second zones without contacting the bottom of the first zones.
The manufacturing method for such a sheet includes embossing the sheet using resiliently clad cylinders by applying enough pressure so that the sheet""s deformation in the zones of the graphics patterns entail, with due account for the papers elastic restoration, a final height which locally exceeds that of the background zone.
Accordingly, the solution of the invention involving the combined graphics and background patterns allows eliminating the additional function of the prior art""s graphics pattern. In particular, the invention attains more deformation at the rim tips of the graphics pattern, achieving thereby enhanced marking in this zone. As a result, pattern contrast is improved. This enhanced marking also opposes partial crushing, for example, when winding the sheet following embossing.